Consumer criteria for food acceptance is complex. Not only sensory characteristics of the product are crucial, but also related factors such as previous information acquired, past experiences, attitude or beliefs are important. Raising concerns regarding ultra-processed food consumption is driving consumers’ willingness towards more traditional and natural products, which are generally considered as safe. Moreover, consumers expect the food industry to supply products with great tasting, convenient and healthy attributes. This is forcing the development of new processing techniques aiming to meet consumer demands (Fig. 1).
High Pressure Processing (also called HPP) is a non-thermal food preservation technology that delivers minimally processed food with extended shelf life. Spoilage microorganisms and pathogens are inactivated by means of cold hydrostatic pressure (up to 600 MPa or 6000 bar), which avoids the use of heat and preserves nutritional, sensory and texture characteristics of the food. The process is applied on already packaged products, eliminating any chance of cross contamination.
The whole sequence of the process is simple (Fig. 2):
1. Already plastic-packaged products are placed in specifically designed containers for the process
2. Containers are pushed into a high-pressure vessel
3. Plugs and wedges close the chamber, which is framed in a metallic yoke. Low-pressure water fills the vessel to remove remaining air
4. Once the high-pressure vessel is completely filled, more water is pumped inside to rise pressure
5. Pressure is held between 1 and 9 minutes (depending on the food)
6. After holding time, pressure is released and the high-pressure vessel opens to remove the processed batch
You may be now wondering how it is possible that packaged food subjected to such a high level of pressure is not completely crushed and destroyed. Well, it has a justification. Water-generated pressure inside the vessel is isostatic, which means that all points of the contact surface of the food receive homogenously and quasi-instantaneously the same pressure intensity, regardless of geometry and size (Fig. 3). To rise pressure up to 600 MPa, water is compressed a 15 %, so volume of food products under this pressure is reduced at least a 15 %. According to Le Chatelier’s principle, a system in equilibrium will shift to a new equilibrium to partially un-do any induced changes. Thus, applying pressure is accompanied by a decrease in volume .
Both, the isostatic and Le Chatelier’s principles explain why integrity of food is maintained after the process. Nutritional and organoleptic properties are also maintained because the isostatic pressure alters distance-dependent molecular interactions, such as hydrogen bonds or electrostatic interactions. In the other hand, covalent bonds are not affected because their working distance cannot be reduced any further . Nutrients and functional compounds such as vitamins, aminoacids or polyphenols and molecules responsible of taste and flavor are stabilized by covalent bonds, so they remain unaffected after the HPP process.
Nonetheless, things are much different for microorganism. Weakening and rupture of big molecules such as proteins or enzymes in the cell membrane or in the cytoplasm induce their inactivation. This accounts for the longer shelf life and increased safety of HPP foods when compared to their unprocessed homologues, but with minimum changes in quality attributes since heat is not applied.
Up to now there are more than 450 industrial HPP machines working worldwide processing an extensive range of foods. The meat and avocado industries were the first ones to benefit from the implementation of this technology, but juices and beverages are gaining importance and represent now the fastest growing sector. Seafood, dairy and ready-to-eat (RTE) industries also account for a large amount of applications. Number of toll processing facilities is steadily increasing. These companies purchase HPP equipment to offer the service to small producers that cannot afford their own HPP unit (Fig. 4).
The benefits that High Pressure Processing offers to the main food areas can be summarized as follows:
Safer products, longer safe life and reduction of chemical additives are the driving forces for sliced and cooked meat products. HPP brings to RTE meat manufacturers the possibility to develop convenient, low salt and additive-free products during chilled storage. Prosciutto, salami or other cured meat producers greatly benefit from HPP when launching their products in countries with strict regulations. In the United States, for instance, food safety authorities established a “zero tolerance” for Listeria monocytogenes, which is recurrent in RTE meats.
Using heat pasteurization to control the pathogen would certainly ruin their delicate sensory attributes of this food category, so HPP becomes an excellent tool to ensure the absence of L. monocytogenes and to open the doors to new markets (Fig. 5). Novel applications in the meat sector include raw pet food. This growing trend poses risks for both, pets and humans, associated with the presence of pathogens. HPP ensures food safety at the same time that food for pets remains raw.
HPP gives the chance to develop safe and premium fruit and vegetable beverages preserving their freshness, color, flavor and nutritional quality. Consumers nowadays demand healthier and fresher juices, which is something that traditional heat pasteurization is not able to offer. For this reason, the number of HPP juice brands has grown significantly in the last 10 years. Volume of HPP juice sold worldwide is estimated in 300 million liters/year, which almost represents 40 % of all HPP products, taking into account that production capacity of many toll processors is almost filled processing juices and beverages for smaller companies.
HPP is able to inactivate spoilage microorganisms, such as molds, yeasts and lactic acid bacteria, which significantly extend shelf life of juices (between 10 to 30 times the shelf life of the fresh product). Nonetheless, nutrients such as vitamins or polyphenols and the delicate molecules that give juices their characteristic colors and flavors remain unaffected since HPP does not break the covalent bonds that makes them stable (Fig. 6). HPP combined with novel juice extraction methods such as cold-pressing delivers premium beverages with extra added value in terms of superior quality and improved nutritional profile.
The trend of fresh and long-lasting products was the main driving force for the massive implementation of HPP in the avocado industry. The untreated fruit and derived products have very short commercial shelf lives (3 to 7 days) due to enzymatic browning and low acidity (pH ranges from 5 to 6), which leads to microbial spoilage and rises food safety concerns. HPP successfully inactivates microorganisms in avocado and avocado products at the same time that significantly reduces enzymatic activity (PPO), which delays browning (Fig. 7).
The application of high pressure processing in this field was the first great success for this innovative technology. Most avocado processors switched from traditional preservation methods (such as freezing, heat pasteurization of addition of chemical preservatives) to HPP aiming to ensure longer shelf lives, freshness and safety of the final products.
In recent years, the number of applications for other vegetable and vegan dips or spreads is increasing considerably. HPP meets consumer demands in terms of flavor and nutritional profile of products such as hummus, tahini, tomato salsas and cheese- or yogurt-based sauces such as tzatziki.
This is probably the most surprising application of HPP technology. Extending shelf life is probably not the main goal in this food sector, but increasing meat extraction yields from crustaceans such as lobsters or crabs at the same time that they remain raw represent the main interest. Easing shell opening of bivalve mollusks such as oysters, clams or mussels is also a major application of HPP.
Pressures between 200 and 300 MPa applied for 30 to 90 s destabilizes the tertiary structure of the protein of the adductor muscle of crustaceans and causes a loss in its functionality: meat is no longer attached to the shell. In addition, the low pressure applied barely modifies properties of seafood, which remains raw. Extracting the meat from tails and claws after the process becomes easier and recovery yields almost reach 100 %.
Culture and tradition are closely related to eating habits. It is then easy to understand why Italians become one of the early-adopters of HPP technology. High quality meat products, delicate flavor and color of fresh juices and Mediterranean diet complement perfectly with HPP and the healthy way of eating of Italians. Cured and fermented meat producers were the first ones to adopt the technology aiming to deliver safe products and gain the ability to export to new markets.
Fruit juice manufacturers followed, so they are now able to maintain quality and nutrients of traditional fruit and berry juices such as pomegranate, aronia or goji. The benefits that HPP offers did not go unnoticed among small producers, so investors saw an opportunity in the tolling business. Multiple companies from various food sectors now use the services that Italian HPP toll processors deliver.
If you wish to know more about the numerous benefits that HPP technology can provide to your products regarding safety and quality do not hesitate to contact Hiperbaric: the global leader manufacturer of HPP equipment for the food industry with more than 10 machines in Italy and presence in the five continents.
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